This invention relates to a novel method for printing a viewing-screen structure for a CRT (cathode-ray-tube), particularly for a multibeam color display tube. The screen structure can be, for example, a light-absorbing matrix or luminescent elements of the viewing screen.
A color television picture tube, which is a type of CRT, comprises an evacuated glass envelope including a faceplate panel having a viewing window, a viewing screen on the inside surface of the window and means for selectively exciting elements of the screen to luminescence. In one type of picture tube, the viewing screen is comprised of interlaced elements having different emission characteristics. Also, the tube includes an apertured shadow mask closely spaced from the viewing screen. The mask is part of the means for selectively exciting the viewing screen, and also is used as a photographic master for depositing the screen structure.
A typical process for fabricating the screen structure includes three photographic exposures, one for defining the elements of each of three different luminescent fields. Each exposure involves projecting a light field from a light source, through a light-refracting lens and a light-transmission filter, incident on a photosensitive layer supported on the inside surface of the viewing window. The lens, the filter and the panel have nominal axes that are substantially parallel to one another. The exposures differ in that the panel is displaced laterally for each exposure relative to the axis of the lens.
Because of the optical characteristics, the brightness of the unfiltered light field drops off from center to edge. To compensate for this, the transmission of the filter increases from center to edge. And, because it is desirable for screen elements to decrease in size from center to edge, the filter produces a brightness profile at the photosensitive layer which produces the desired distribution of screen-element sizes. That distribution is substantially symmetrical around the nominal center of the viewing screen.
Ordinarily, the light source and all of the axes fall on a common axis for one exposure. And, for the two other exposures, only the panel is displaced from this relationship so that the panel axis is on one side and then on the opposite side of the common axis. It has been observed that these two other exposures define screen elements that are larger on one side of the panel than on the other side of the panel. The novel method overcomes this problem by appreciating the cause of this problem and specifying a nonobvious repositioning of the filter in the two other exposures.